1. Field of the Invention
The invention, in general, relates to a ring oscillator and, more particularly, to a ring oscillator of the kind which is insensitive to variations in supply as well as substrate voltages to reduce jitter resulting from such voltages.
2. The Prior Art
For the reduction of jitter in a ring oscillator, WO 96/38913 and U.S. Pat. No. 5,559,476 suggest the use of current-controlled delay elements to isolate the influence of the supply noise on the stability of the frequency. Such elements are disadvantageous as they must be used for each stage of the ring oscillator. Furthermore, they require a control voltage the noise of which produces jitter.
U.S. Pat. No. 5,418,498 discloses a ring oscillator said to provide low jitter by utilizing gates and devices with time delays of excellent stability. However, the circuit is relatively complex.
EP0 805 553 A1 discloses an oscillator said to yield low jitter as a result of negative frequency feedback. Since it requires a second ring oscillator to provide the feedback, its disadvantages are rather obvious. Moreover, the second ring oscillator significantly increases the complexity of the circuit.
"Oscillator Jitter Due to Supply and Substrate Noise, IEEE 1998 Custom Integrated Circuits Conference, pp. 489-492" describes an alleged design optimization for MOS ring oscillators. However, jitter is only partly reduced.